The invention relates to the field of digital-to-analog converters (DAC), and in particular to combining a continuous-time stage with a switched-capacitor stage for digital-to-analog converters (DACs) and low-pass filters.
Audio signals are increasingly stored as digital audio signals on CDs, computer hard-drives and portable music players such as MP3 devices. The stored digital signals are typically stored as a series of digital word samples of the original analog sound. Such digital audio signals need to be converted into corresponding analog signals for application to audio transducers such as loudspeakers and earphones, and often amplification, mixing, switching, or filtering stages as well.
Various methods of achieving this conversion while maintaining low distortion and other desirable characteristics are known. FIG. 1A illustrates an approach where the digital N-bit sampled signal from a digital source 1 such as a CD player in put directly to a N-bit DAC 2 comprising a resistor network. This gives an intermediate analog signal quantized to 2N voltage levels and sample-and-held at the audio sample rate; which signal is then passed through a low-pass filter 3a to give a smoothed analog waveform. Since the audio sample rate is typically only 44.1 kHz or 48 kHz a fast roll-off filter 3a is required to pass signals at 20 kHz but attenuate signals at say 44.1 kHz−20 kHz=24 kHz in order to avoid aliasing effects. Also the DAC 2 needs to have the full resolution of the digital word, so to get say—100 dB distortion, one needs a DAC 2 linear to better than 16 bits, which is difficult and expensive and power-hungry. However, for lower-performance applications, say, telephony, this technique may be appropriate. As noted, the DAC may comprise a resistor string and selection switches as drawn, or alternatively, an array of selectable parallel current sources, or switched capacitor circuitry as is known.
FIG. 1B illustrates an alternative scheme. The N-bit digital word from the digital source 1 is input to a digital delta-sigma modulator 4, which outputs a single-bit digital stream, at a much higher sample rate, for example 6 MHz. The quantization noise introduced is spectrally shaped to move it out of the audio band. The DAC 4 now need only be a simple 1-bit DAC, essentially a switch between two voltage levels. Its two-level output then requires filtering, but the anti-aliasing requirements are much more relaxed given the higher sample rate, and so a simpler, cheaper filter 3b can be implemented. In implementations the DAC switches may drive voltages into resistors of the following filters, or control the switching of capacitors in switched-capacitor filters, or may control current sources feeding into following filters.
FIG. 1C illustrates a further alternative, in which a multi-bit (n-bit) delta-sigma converter 6 is used. The multi-bit output has less in-band quantization noise than the single-bit implementation, allowing better performance for a given clock frequency, and is also less sensitive to clock jitter than a 1-bit scheme. A resistor network DAC 7 can be used as shown to generate an n-bit sampled analog intermediate signal, though in practical implementations switched-capacitor or switched-current circuitry may be used. The intermediate waveform, prior to the smoothing filter 3c, has less out-of-band quantization noise so further easing the requirements on this filter. Because of the higher sample rate, known techniques such as dynamic-element matching can be used to move distortion components due to mismatch of component elements of the n-bit DAC 7 out of the audio band.
The term “delta-sigma”, as used here in “delta-sigma modulator” or “delta-sigma converter” or “delta-sigma bit stream” for instance, are taken here to encompass any similar scheme, such as “noise shapers”, which take a lower-sample-rate wide-bit-width data input data stream and output a high-sample-rate small-bit-width data stream, usually but not necessarily shaping the spectrum of the resulting extra quantization noise so that it falls outside the signal band of interest.
In each of the above systems there is a need for a DAC (2, 4, 6), either single bit or multi-bit, and also a need for a filter (3a, 3b, 3c) for smoothing of the output waveform. Typically today the DAC and filter are implemented using CMOS integrated circuit technology. The DAC and filter may be implemented using switched-capacitor techniques, although the some or all of the filtering may be implemented just using a CMOS op amp and passive resistors and capacitors, on-chip or off-chip.